Duncan Lyster

Abstract:

Abstract Comparing observed and predicted surface temperatures on airless planetary bodies is crucial for interpreting their surface processes, constraining activity, and deriving their surface properties. We present a new tool in this endeavour: TEMPEST (Thermophysical Evolution Model for Planetary Environment Surface Temperatures). It is a modular, open-source Python model for diurnal thermal evolution on irregular bodies. TEMPEST can reproduce non-Lambertian multiple scattering using arbitrary Bidirectional Reflectance Distribution Functions (BRDFs) and notably incorporates surface roughness on sub-facet scales through representative sphere-section geometry. The model includes an explicit time-stepping one-dimensional periodic conduction solver influenced by Spencer’s thermal model Thermprojrs, and a semi-implicit Crank-Nicolson solver, and can be paired with other thermal solvers. Precomputed look-up tables enable fast application of scattering laws (Lambertian, Lommel-Seeliger, Hapke, or input empirical BRDF data), allowing the user to assess their effects in permanently shadowed regions or partially shadowed terrains. TEMPEST is parallelised, running on personal computers in minutes for thousands of facets and scaling to high-performance servers for models exceeding 100,000 facets. Validation was performed against Thermprojrs in one dimension, against Sorli’s Binary Thermophysical Model in three dimensions, and with empirical thermal data from the Lunar Reconnaissance Orbiter, showing good agreement in each case. Application to the high-resolution shape-model of 25143 Itokawa shows the importance of self-shadowing and radiative mutual heating in controlling local temperatures. TEMPEST has already been used to analyse data from the Lucy Mission, Galileo, and ground-based observations, and is adaptable to upcoming datasets from targets of missions such as Comet Interceptor and Europa Clipper.

Abstract:

Abstract We investigate the diurnal temperature variations of Europa’s surface using brightness temperatures taken by Galileo’s Photopolarimeter-Radiometer (PPR) instrument. Diurnal curves created from these data are compared to those predicted by a thermophysical model to determine what thermal inertias and Bond albedos can fit the data within a reduced chi-squared cutoff of $\chi _{\textrm {red}}^2\le 1.0$. This analysis is used to extensively quantify the uncertainty for the first time of the two thermophysical parameters derived from PPR. We map the albedo and thermal inertia for 33% and 24% of Europa’s surface area, respectively. We find a range of 0.375-0.75 for albedo and 20-110 J m−2 K−1 s−1/2 for thermal inertia. Our uncertainty analysis indicates well-constrained estimates for albedo, while upper limits for thermal inertia remain poorly constrained. When averaged across nine geological areas, albedo varies as expected: lower values are obtained for darker regions in visible wavelengths. Thermal inertia appears to vary independently from geological boundaries. The surface was also divided based on electron bombardment energy flux and non-ice composition fraction. Thermal inertia behaves opposite of what is expected: lower thermal inertias are located in regions of high-energy electron bombardment. This possibly indicates a mechanism competing with electron-induced sintering is present that is lowering the surface conductivity. No endogenic anomalies were detected that could not be explained by passive emission alone. Nevertheless, these results aid in preparing for future thermal measurements from Europa Clipper and Juice by improving the surface coverage of Europa’s passive thermal properties and providing uncertainties of their values.